Kiwi genomes explain the unusual characteristics of an endangered bird

The kiwi bird’s unique nocturnal behaviour is linked to some altered genes that eliminate color vision and others that modify its sense of smell, according to the first kiwi genome published in the open access journal Genome Biology.

Kiwis are endangered, ground-dwelling birds endemic to New Zealand. They are the smallest and only nocturnal representatives of the ratites – a group of flightless birds that includes the ostrich and emu. They are also unusual in that they have a highly developed sense of smell, low metabolic rate, and enormous eggs in relation to body size. However, the genetic adaptations that underpin their unique traits have so far not been well understood.

A team of researchers sequenced the genomes of two North Island brown kiwi. Not only was the kiwi genome found to be one of the largest bird genomes sequenced to date, but the team also identified evolutionary changes in its genome that help explain the bird’s unique adaptations to nocturnality – a behavior found in fewer than 3% of all bird species.

Lead author Diana Le Duc from the University of Leipzig and the Max Planck Institute for Evolutionary Anthropology, Germany, said: “We’ve seen for the first time that kiwi lack color vision, and that their olfactory receptors can probably detect a larger range of odors which may be essential for their night-time foraging. These adaptations seem to have happened around 35 million years ago, soon after their arrival in New Zealand, probably as a consequence of their nocturnal lifestyle.”

The gene responsible for black and white vision, rhodopsin, was found to be similar to other vertebrates. However, the team identified mutations in the green and blue vision receptor genes, which could render blue and green color vision absent in the kiwi.

The genomic changes in kiwi vision and smell are consistent with changes that are thought to occur during adaptation to nocturnal lifestyle in mammals. The researchers estimated the onset time of these changes to around 35 million years ago, suggesting that the kiwi adopted its nocturnal lifestyle shortly after the arrival of its ancestor in New Zealand. At the time the kiwi arrived other ratites, the moa, already inhabited New Zealand. These now extinct birds, of which one species was over three metres in height, are thought to have monopolized food sources during the day, forcing the kiwi to adopt an alternative nocturnal lifestyle.

Kiwi are unique among birds in having nostrils present at the end of their long beaks and it has long been thought they are more similar to mammals than birds in their reliance on tactile and smell senses for foraging. The kiwi genome showed significantly higher diversity in smell receptors than other investigated birds, suggesting that they may be able to distinguish a larger range of odors.

Nocturnal animals tend to have low energy metabolism, and kiwi have the lowest metabolic rate among all birds. In the genome, the team found enriched changes in genes related to energy expenditure, reserves and metabolic processes, which may also be linked to this nocturnal lifestyle.

Diana Le Duc added: “Despite conservation efforts, North Island brown kiwi are still at high risk of extinction. We made a first estimate of the diversity of the kiwi genome by comparing the sequence of two individuals, and it appears to be as low as that of inbred birds. This is an important indication of the level of the threat, and we expect further insights from the genome to help in developing conservation management strategies.”

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